The idea of using an injectable material for soft tissue augmentation developed soon after the invention of the hypodermic needle. Various products have been injected into the human body for correction of soft tissue defects including paraffin, petrolatum, vegetable oils, lanolin, bees wax, silicone, and more recently, collagen.
Paraffin was first used by Gersury in 1899.sup.1 who injected it into the scrotum of a young man to replace resected testicles. He later used paraffin to correct facial contour defects. During the period of 1900-1914, the use of paraffin injections became popularized.
Heidingsfeld in 1906.sup.2 described the paraffinoma. Injected paraffin forms many widely diffused droplets in the tissues. Phagocytosis by macrophages and giant cells occurs, followed by hyalin necrosis of fibrovascular septa, proliferation of fibroblasts, and development of scar tissue containing oil globules and cysts lined by foreign body giant cells.
Clinically, edema and scar formation occurred, sometimes followed by ulceration. The use of injectable paraffin was discontinued in the United States around World War I, but continued to be used in the Far East until the 1950's.
Silicones were the next material to be injected into humans on a large scale. Conway and Goulian reported on the use of silicone injections into the breast and face in 1963.sup.3. Some people were using the "Sakurai" formula, a silicone fluid, adulterated with an additive for better fixation.sup.4. Dow Corning developed a more purified "medical grade 360 liquid silicone" and, although this product was not originally intended to be injected, it was soon used for human injections around the world.
The first reports of complications of injected silicone including foreign body granulomas occurred in 1964.sup.5. In 1965, Ben-Hur and Neuman.sup.6 described the siliconoma which was a tumor-like formation developing after the injection of silicone 360 in rats.
In 1965, the U.S. Food and Drug Administration (F.D.A.) determined that the clinical use of injectable silicone was a "drug use" and authorized seven investigators to employ silicone fluid as a soft tissue substitute. After reviewing their clinical experiences with injectable silicone - Dow Corning - MDX 4.4011, they concluded that silicone MDX 4.4011 can provoke an inflammatory reaction resulting in redness and ecchymosis which can be controlled with antibiotics and corticosteroids. Partial resorption and migration can occur but can be avoided by repeated injections of small amounts.sup.7,8,9,10,11,12,13. Although few complications have been reported with MDX 4.4011, many complications have been described after injection of liquid silicone with an unknown grade of purity.sup.14. Although a few individuals are still injecting silicone with good results and few complications, the unforgiving nature of the material if used incorrectly will probably prevent its widespread use.
Bovine collagen has recently gained widespread use as an injectable material for soft tissue augmentation.
Collagen is the principal extracellular structural protein of the animal body. At least seven types of mammalian collagen have been described. Their common characteristic is a three stranded helix, consisting of 3 polypeptide chains, called alpha-chains. All chains have the same configuration, but differ in the composition and sequence of their aminoacids. This leads to different types of alpha chains, but they all have glycine in every third position of the aminoacid sequence. This allows the helical conformation to occur. Type I collagen is composed of 2 alpha.sub.1 -chains and one alpha.sub.2 -chains and is the principal extracellular material of skin, tendon, and bone. When clinicians speak of "collagen," they are usually referring to Type 1. Type II collagen is found in cartilage and the vitreous humor. Type III collagen is present in rapidly growing tissue, particularly juvenile and healing skin. Collagen Types IV and V are found in epithelial basement membrane.sup.14.
The major molecular species beside collagen that are found in the extracellular matrix include the noncollagenous structural glycoproteins, elastin, and the proteoglycans. The structural glycoproteins consist of fibronectin and laminin. Fibronectin is found in both the plasma and tissue forms and is capable of interacting with other components of the extracellular matrix. Recently, Wartiovaara proposed that another function of fibronectin is to opsonize collagen or fibrin and, by this mechanism, to regulate the cellular digestion of these substrates.sup.15. Laminin is found in all basement membranes. Proteoglycans are characterized by a protein core linked to glycoaminoglycan side chains.sup.15,16,17,18.
When using collagen as a biomaterial, it is important to use it in its purest and crystalline form to eliminate the noncollagenous proteins that are far more potent antigens. Once the inflammatory cycle is stimulated, the resorption of collagen occurs by the infiltrating inflammatory cells, principally macrophages and, to a lesser extent, granulocytes. These cells contain collagenase which acts to digest collagen.sup.19. Houch and Chang demonstrated that skin collagen was chemotactic itself and became even more active by digestion with tissue collagenase into smaller peptide fragments.sup.20. Chemotropism is the attraction of living protoplasm to chemical stimuli whereby the cells are attracted (positive hemotaxis) or repelled (negative chemotaxis) by acids, alkalis or other bodies exhibiting chemical properties. Postlethwaite et al.sup.21 showed that various types of collagens, their alpha-chains, as well as small peptides formed by collagenase digestion were all chemotactic to dermal fibroblasts. They concluded that the chemotactic migration of fibroblasts into the site of tissue injury or theoretically injected collagen can be regulated by the solubilized collagen or its degradation products. Thus, a collagen implant would not remain dormant in the tissue but a complex series of events may occur. First, the collagen implant could be invaded by inflammatory and fibroblasts and, while being continuously resorbed, it could promote an inflammatory reaction by chemotactic properties of its degradation products. Thus, the area of collagen metabolism is not only important for collagen and other soft tissue injectable materials, but also to both normal and abnormal wound healing (i.e. hypertrophic scarring and keloids).sup.19,20,21.
The injectable collagen that recently gained widespread use was given marketing clearance as a device (not a drug) by the Food and Drug Administration in 1981. This material sold under the name Zyderm.RTM. Collagen Implant is a purified bovine collage. About 95% consists of Type I collagen and the remaining 5%, Type III collagen. The collagen undergoes proteolytic hydrolysis of its telopeptide portion to decrease its antigenicity. The material is suspended in physiologic saline buffer, and 0.3% lidocaine is added and the material is packaged in syringes ready for injection through small gauge needles.
The most immediate concern to most plastic surgeons is the fate of bovine collagen after injection. Zyderm I.RTM. with 35 mg/ml of collagen is rapidly degraded by tissue collagenases and resorbed within months. Zyderm II.RTM. with 65 mg/ml of collagen and, thus, almost twice the concentration of collagen, is longer lasting but follows the same fate as Zyderm I.RTM.. Zyplast.RTM. was most recently introduced containing 35 mg/ml of collagen cross-linked with glutaraldehyde. Zyplast.RTM. also is ultimately degraded over time. Kligman.sup.22 recently compared the biological fat of Zyderm.RTM. collagen and Zyplast.RTM. collagen when implanted into the back of human volunteers. They reported that, while Zyderm.RTM. collagen was apparently resorbed by host tissue within months of implantation, Zyplast.RTM. was more persistent. Fibroblasts infiltrated the Zyplast.RTM. collagen and deposited host collagen. Burke et al.sup.23 reported that Zyderm.RTM. collagen stimulates a response which results in implant degradation and replacement by newly generated host collagen.
One additional area of confusion about Zyderm I.RTM. collagen, Zyderm II.RTM. collagen and Zyplast.RTM. collagen is the percentage of collagen in each. Zyderm I.RTM. collagen and Zyplast.RTM. collagen have 35 mg/ml, (3.5% collagen) while Zyderm II.RTM. has 65 mg/ml (6.5% collagen).
A small percentage of patients receiving either Zyderm I.RTM. collagen, Zyderm II.RTM. collagen or Zyplast.RTM. collagen, hereafter referred to as bovine collagen implants (BCI), have developed adverse reactions of an immune nature. The safe use of these implants is based on the reported low immunogenicity of the bovine collagen. It is, however, contraindicated in patients with a history of autoimmune disease. Skin tests are required before receiving the BCI. Only patients with negative skin tests after 4 weeks should have the treatment injections. The Collagen Corporation indicates that approximately 3% of the patients have a positive skin test reaction characterized by edema, induration, erythema, pruritus or tenderness at the injection site. Adverse generalized treatment reactions have been quoted from less than 1% to greater than 5%. They are characterized by urticaria, myalgias, arthralgias and one anaphylactoid reaction .sup.23,24. A dramatic increase in the incidence of anti-BCI antibodies in the circulation of patients with adverse BCI-treatment reactions has been noted compared with serum samples from untreated individuals or treated patients suffering adverse reactions. BCI is a weak antigen but still is a foreign protein. It has been treated in such a way to cleave the telopeptides to make it less immunogenic but the helical portion of the molecule retains its antigenic loci. The dominant structures recognized by the cell-mediated immunological mechanism appear to reside within the triple helical body of the collagen molecules. There has been a major concern about repeated exposure of patients to these antigens and their long-term effects.sup.24,25,26.
In summary, due to the shortcomings of the BCI, including the lack of persistence, need for repeated injections and serious concern over adverse reactions, newer injectable materials for soft tissue augmentation are needed. The present invention is directed to injectable materials for soft tissue augmentation and their methods of manufacture which overcome shortcomings of BCI and other injectable materials of the prior art.